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Investment Securities
 12 Months Ended
Dec. 31, 2020
Investments, Debt and Equity Securities [Abstract]  
Investment Securities Investment Securities 
 
The following tables present the amortized cost, unrealized gains, unrealized losses and approximate fair values of debt securities at December 31, 2021 and 2020: 

December 31, 2021
(in thousands)Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:    
U.S. Treasury and agencies$894,969 $27,279 $(4,195)$918,053 
Obligations of states and political subdivisions320,338 11,734 (1,288)330,784 
Residential mortgage-backed securities and collateralized mortgage obligations2,649,048 19,093 (46,543)2,621,598 
Total available for sale securities$3,864,355 $58,106 $(52,026)$3,870,435 
Held to maturity:    
Residential mortgage-backed securities and collateralized mortgage obligations$2,744 $770 $— $3,514 
Total held to maturity securities$2,744 $770 $— $3,514 

December 31, 2020
(in thousands)Amortized CostUnrealized GainsUnrealized LossesFair Value
Available for sale:
U.S. Treasury and agencies$698,243 $64,271 $(312)$762,202 
Obligations of states and political subdivisions263,546 15,996 (31)279,511 
Residential mortgage-backed securities and collateralized mortgage obligations1,839,711 51,583 (449)1,890,845 
Total available for sale securities$2,801,500 $131,850 $(792)$2,932,558 
Held to maturity:    
Residential mortgage-backed securities and collateralized mortgage obligations$3,034 $849 $— $3,883 
Total held to maturity securities$3,034 $849 $— $3,883 

The Company elected to exclude accrued interest receivable from the amortized cost basis of debt securities disclosed throughout this note. Interest accrued on investment securities totaled $10.4 million and $8.9 million as of December 31, 2021 and 2020, respectively, and is included in Other Assets.
Debt securities that were in an unrealized loss position as of December 31, 2021 and 2020 are presented in the following tables, based on the length of time individual securities have been in an unrealized loss position.
 
December 31, 2021
Less than 12 Months12 Months or LongerTotal
(in thousands)Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$197,529 $2,749 $28,378 $1,446 $225,907 $4,195 
Obligations of states and political subdivisions75,200 1,153 2,162 135 77,362 1,288 
Residential mortgage-backed securities and collateralized mortgage obligations1,777,288 40,579 129,943 5,964 1,907,231 46,543 
Total temporarily impaired securities$2,050,017 $44,481 $160,483 $7,545 $2,210,500 $52,026 

December 31, 2020
Less than 12 Months12 Months or LongerTotal
(in thousands)Fair ValueUnrealized LossesFair ValueUnrealized LossesFair ValueUnrealized Losses
Available for sale:      
U.S. Treasury and agencies$29,493 $312 $— $— $29,493 $312 
Obligations of states and political subdivisions4,357 31 — — 4,357 31 
Residential mortgage-backed securities and collateralized mortgage obligations215,165 449 — — 215,165 449 
Total temporarily impaired securities$249,015 $792 $— $— $249,015 $792 

These unrealized losses on the debt securities held by the Company were caused by changes in market interest rates or the widening of market spreads subsequent to the initial purchase of these securities and are not due to the underlying credit of the issuers. Management monitors the published credit ratings of the issuers of the debt securities for material rating or outlook changes. Substantially all of the Company's obligations of states and political subdivisions are general obligation issuances. All of the available for sale residential mortgage-backed securities and collateralized mortgage obligations portfolio in an unrealized loss position at December 31, 2021 are issued or guaranteed by government sponsored enterprises. Because the decline in fair value of the debt securities is attributable to changes in interest rates or widening market spreads and not credit quality, these investments do not have an allowance for credit losses at December 31, 2021.

The following table presents the contractual maturities of debt securities at December 31, 2021:
Available For SaleHeld To Maturity
(in thousands)Amortized CostFair ValueAmortized CostFair Value
Due within one year$11,608 $11,749 $— $— 
Due after one year through five years250,556 256,279 2 2 
Due after five years through ten years979,600 1,004,824 7 7 
Due after ten years2,622,591 2,597,583 2,735 3,505 
Total securities$3,864,355 $3,870,435 $2,744 $3,514 
The following table presents, as of December 31, 2021, investment securities which were pledged to secure borrowings, public deposits, and repurchase agreements as permitted or required by law: 
(in thousands)Amortized CostFair Value
To state and local governments to secure public deposits$271,285 $276,094 
To secure repurchase agreements610,093 625,543 
Other securities pledged 244,520 245,794 
Total pledged securities$1,125,898 $1,147,431